When zeta potentials of liposomes formed by phosphatidylcholine (PC) and st
earylamine (STE) at variable concentrations were compared with those corres
ponding to liposomes formed by phosphatidic acid (PA), a decline and a lack
of linearity in their zeta potentials compared to the logarithm of ionic s
trength were found at concentrations of STE above 10% (molar ratio). Despit
e the fact that STE is fully protonated at the interval of pH used,(2-8) ze
ta potentials were found to be dependent not only on the ionic strength of
the medium, as predicted by the classical double-layer theory, but also on
the pH. Determination of the STE distribution by spectrofluorometry followi
ng the labeling of STE with fluorescamine showed that STE seemed to be pref
erentially located in the outer monolayer. This apparent contradiction can
be explained by the migration of STE molecules from the liposomal surface t
o the medium, where it is organized in the form of micelles with a diameter
of about 2 nm. The presence of micelles in addition to liposomes involves
a large adsorption-desorption equilibrium, which, in turn, is influenced by
the variation in the electrostatic free energy of the double layer on the
membrane. Thus, the surface charge density varies with the change in ionic
strength and pH, and consequently, the electrophoretic behavior of STE lipo
somes differs from that of PA liposomes.